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Why the text terminal cursor is important for Accessibility

Posted on June 25, 2021
Tags: Accessibility, brick, Haskell, vty

Most terminal user interfaces can be used by blind users pretty
easily. But why is that, actually? After all, the content of a
terminal is pretty much unstructured data. A rectangular grid of
characters with no extra information except colours. But is that
really true?

The most important bit of metadata for a terminal screen reader is
actually the cursor location. This is the one bit of information that
is required to make a unstructured grid of characters surprisingly
useful to a person mostly constraint to linear reading.

The terminal cursor acts as the focus. It indicates which part of the
screen is currently being edited or selected. For editors, this is a
pretty well understood concept. Where the cursor is, the next
character will be inserted. However, there is more to a text-based
user interface then just editing fields. And the cursor is not always
visible.

Enter brick: a TUI for Haskell

A while ago, I looked at the brick terminal UI library for Haskell.
When I tried its various demo programs, I noticed that my screen
reader was reporting the cursor being in the lower right corner of
the application when a menu item was selected. I had to manually
investigate the screen and look at attributes (colours) to figure out
which item in a list was currently selected. Amongst BRLTTY
developers, we've decided long ago to not work around these issues on
the screen reader side, rather try and make use of open source and
fix the problems whenever we see them in the wild. Behind the scenes,
we have fixed a bunch of frameworks and applications to place the
cursor at the locus of focus. So I set out to understand brick
internals to fix this.

A collection of cursors

brick works different from most TUI frameworks I know. You don't have
a single cursor which you set to a particular location. Rather, all
the different components drawn on the screen can declare their own
cursor location, and the composition mechansim ultimately chooses
which cursor should be used.

This, while being pretty flexible, looked fundamentally wrong to me.
Why? It doesn't reflect the reality of a terminal.

Der Cursor ist immer unter uberall!

There is no such thing as "no cursor" in a terminal. The cursor might
be hidden, so it is not rendered on the screen. But the cursor still
has a location where it sits and waits to print the next output
character to the screen. A screen reader will pick that location up,
no matter if the cursor is visible or hidden.

Visibility is key

So after patching brick to declare a cursor when rendering certain
list items, checkboxes and radio items, I realized the actual missing
bit. brick had no concept of cursor visibility. Rather, if the
composition mechansim did not see a cursor declared, it would hide
the cursor on-screen and "pretend" there was none. However, as we
have learnt above, thats just not true and programs like screen
readers actually rely on the cursor location to indicate the locus of
fucs.

Raising this issue with the brick maintainer uncovered the fact that
the low level vty library used by brick to do the actual terminal
output did not have a concept of a hidden cursor either. Jonathan
fixed this in vty 5.33.

Tying the knot

And since vty 5.33 is now in stack LTS, I thought to myself yesterday
it is time to finally add support for invisible cursors to brick.

There is now a new function putCursor which has the same type
signature as the already existing showCursor, but will make sure the
cursor is not visible on-screen. This can and should be used to place
a cursor at the locus of focus, even if that location is already
visually indicated by different attributes.

The stock widgets that come with brick should now all be screen
reader friendly. If you happen to maintain a brick application which
provides a render function to something like renderList, please
consider extending it to use showCursor. Once brick 0.64 is released,
you can change to putCursor to clean up the visual appearance of your
program.

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